This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
The present invention relates to a valve assembly, in particular to a shut-off flow valve assembly. The valve assembly of the present invention finds particular use in wellhead assemblies and the control of fluids produced from subterranean wells, in particular in subsea locations, for production and process control.
Conventional and known valve assemblies may be divided into several categories. Chokes are widely used to control or adjust the pressure of a fluid stream. For example, generally wellhead assemblies will comprise one or more chokes to reduce the pressure of fluids produced from the well, in order to match the pressure of the fluid downstream of the choke and wellhead assembly to the requirements of the downstream systems and installations. Chokes are typically characterized by being slow acting and providing little or no flow shut-off capabilities. Accordingly, chokes must be used in conjunction with other valve assemblies in order to provide a capability of shutting off the flow of fluid completely, for example when it is needed to isolate the production of fluids from a well. A very common design of choke is the so-called ‘plug and cage’ arrangement or the so-called ‘sleeve and cage’ arrangement, in which the pressure of the fluid stream is adjusted by causing the fluid to flow through apertures in a generally cylindrical cage. A plug or sleeve is disposed either inside or outside the cage and is moveable longitudinally with respect to the cage, to reveal or close apertures in the cage in order to achieve the desired pressure of fluid downstream of the choke assembly.
Valves of differing designs and operating principles are also known. For example gate valves and ball valves are known. These forms of valve are known for use in isolating or directing fluid flow and are generally operated between a fully open position and a fully closed position. They offer some ability to control the flow of fluid between the fully open flowrate and zero flow achieved with the valve in the fully closed position. However, they are generally only operated at relatively low pressure differentials. Gate valves and ball valves are generally only operated where no significant pressure differential exists between the fluid upstream and downstream of the valve. An alternative form of valve is the butterfly valve, generally also operated in a fully open or fully closed position, to control fluid flow. As butterfly valves are generally efficient only at low pressures, they are seldom used in wellhead installations, where a valve must be able to cope with being exposed to fluid at full wellhead pressure.
Check valves are used to allow fluid flow in a given direction, typically once a predetermined threshold pressure has been achieved, but to prevent the flow of fluids in the reverse direction. Various designs of check valve are known and operated, including ball check valves, diaphragm check valves and swing check valves. However, these valves are generally not suitable for use as shut-off valves to prevent fluid flow.
There is a need for an improved valve assembly, in particular for use in the control of fluids produced from a subterranean well, for example from a wellhead into a production and processing assembly.